The present invention relates to a method for matching the direct voltage control unit of an inverter station to the control path, in a system in which the inverter station includes n thyristor bridges connected in series at their direct voltage ends and connected to a three-phase mains via n individually associated variable ratio transformers, n extinction angle control circuits for the individual thyristor bridges, and a total control circuit for controlling the voltage of all thyristor bridges together with respect to voltage. In this system, the output value of the common control circuit constitutes the direct voltage of the inverter station and its independent variable is the transformer stage adjustment which operates in synchronism for all thyristor bridges. The invention also relates to an apparatus for practicing the method.
In the normal use, the direct voltage of a pole of a high voltage direct current transmission system is set in the inverter station. As already noted, this station includes a series connection of n thyrister bridges, each bridge having associated with it a transformer which is variable in stages so as to provide voltage matching. The individual transformers can be controlled in synchronism. If the thyristor voltage drop and the ohmic voltage drops are neglected, the direct voltage U.sub.WR for the inverter station is represented as follows, where .xi. is the error in the transformer stages from the desired value, with synchronism of the transformers and with the same commutation reactances: ##EQU1## n being the number of thyristor bridges in operation, U.sub.S the secondary voltage at the transformer during idling, .gamma..sub.j (with j=l . . . n) the extinction angle of the individual thyristor bridges, I.sub.d the average direct current value, I.sub.dN the desired direct current value, and D.sub.x the change in the inductive direct voltage.
The extinction angle .gamma..sub.j of each thyristor bridge is regulated separately. In this case, control is effected via the grid control of the thyristor devices. The direct voltage of all thyristor bridges, i.e. of the total system, is regulated by means of a three-point regulator acting on the transformer stages. The three-point regulator supplies one of the three signals "higher", "lower" or "steady" to the transformer stage adjustment. The stage adjustment of the transformers for the bridges in operation is done in synchronism.
Due to this adjustment in discrete and constant steps and for the reason of stabilization the control circuit is affected by a remaining error even in the steady state.
Each bridge can be switched in or out of operation by means of its associated grid control during operation of the system. As soon as a bridge is switched out of operation an associated by-pass switch is closed. The control parameter for the direct voltage regulation via the transformers must then be adapted accordingly. At the same time the circuit gain of the control circuit must also be matched. That gain is determinative of the remaining error in the steady state. The error should be as small as possible.
In principle, such matching can be effected with the aid of return signals from defined switch positions at the beginning or at the end of each switch-on or switch-off operation of a thyristor bridge, e.g. from the by-pass switch. Since the process can extend over a longer period of time, due to the behavior of the system, it is inevitable that errors occur in the stages.